Recently, with increasing integration degree of a semiconductor device, dimensions of individual element become finer, and widths of wiring and gate constituting each element also become finer.
A process of transferring an original plate (a mask or a reticle, hereinafter collectively referred to as a mask) to a photosensitive resin to fabricate a wafer is fundamental to production of a semiconductor integrated circuit. The semiconductor integrated circuit is produced by repeating the fundamental process.
An exposure apparatus called a stepper or a scanner is used in the transfer process. In the exposure apparatus, light is used as a transfer light source, and a circuit pattern on the reticle is projected onto the wafer while reduced to about one-fourth to about one-fifth. In order to increase the integration degree of the semiconductor integrated circuit, it is necessary to improve resolution performance in the transfer process. If NA is a numerical aperture of an imaging optical system, and λ is a wavelength of the light source, a resolution dimension is proportional to (λ/NA). Accordingly, higher exposure resolution can be achieved by increasing the numerical aperture NA or decreasing the wavelength λ.
As another example for the higher exposure resolution, nanoimprint lithography (NIL) has attracted attention as a technology for forming the fine pattern. In the nanoimprint lithography, a fine pattern is formed in a resist by pressuring a master template (a mold) having a nanometer-scale fine structure to the resist on the wafer. In the nanoimprint technology, in order to enhance productivity, plural duplicate templates (replica templates) are produced using a master template that is an original plate, and then the replica templates are attached to and used in each nanoimprint lithography apparatuses.
It is necessary to improve a production yield of the expensive LSI in a production process. A defect of a circuit pattern formed on of a mask or template can be cited as a large factor that reduces a production yield of the semiconductor element. It is necessary to detect the shape defect of the extremely small pattern in a mask inspection process. Japanese Patent Number 4236825 discloses an inspection apparatus that can detect fine defects in the mask.
In the mask inspection process, the mask is illuminated with the light while the mask is moved with a mask stage, and the pattern formed on the mask is imaged with an imaging element such as a CCD camera. Then, an obtained optical image is compared to a reference image, namely, an image that is compared to the optical image of a pattern in order to detect a defect, and a place where a difference between the optical image and the reference image exceeds a threshold is detected as a defect. The difference, for example, can be caused by short defects or open defects.
In the inspection, it is necessary to sufficiently ensure a quantity of light incident to the sensor. A shortage of the light quantity causes degradation of inspection accuracy and lengthening of inspection time. In a reflected illumination optical system, light emitted from a light source is reflected by a half mirror, a mask is irradiated with the light, the light reflected by the mask is transmitted through the half mirror, and the light is incident to a sensor to capture an optical image. At this point, only the light reflected by the half mirror is used as the illumination light for the mask, which decreases the light quantity to a half of the quantity of light emitted from the light source. Then, only the light transmitted through the half mirror in the light reflected from the mask is used as the light incident to the sensor, which further decreases the light quantity to a half again. That is, in the reflecting optical system, the light incident to the sensor becomes a quarter of the quantity of light emitted from the light source.
Therefore, there is a demand for an image capturing device that can capture an image of a mask pattern while minimally restraining the degradation of the light emitted from the light source in the reflected illumination optical system, and inspection apparatus or an inspection method. The invention has been devised to solve the problem described above. An object of the invention is to provide an image capturing device that can minimally restrain the degradation of the light quantity in the reflected illumination optical system to capture an image of a target. Another object of the invention is to provide an inspection apparatus and an inspection method, for being able to minimally restrain the degradation of the light quantity in the reflected illumination optical system to perform high-accuracy inspection.
Other advantages and challenges of the present invention are apparent from the following description.